linux-kernel - Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS

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Date:   Tue, 23 Jul 2019 23:59:54 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "songliubraving@...com" <songliubraving@...com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
CC:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "hdanton@...a.com" <hdanton@...a.com>,
        "kernel-team@...com" <kernel-team@...com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH v9 5/6] mm,thp: add read-only THP support for
 (non-shmem) FS

On Mon, 2019-06-24 at 17:12 -0700, Song Liu wrote:
> This patch is (hopefully) the first step to enable THP for non-shmem
> filesystems.
> 
> This patch enables an application to put part of its text sections to THP
> via madvise, for example:
> 
>     madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
> 
> We tried to reuse the logic for THP on tmpfs.
> 
> Currently, write is not supported for non-shmem THP. khugepaged will only
> process vma with VM_DENYWRITE. sys_mmap() ignores VM_DENYWRITE requests
> (see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is
> execve(). This requirement limits non-shmem THP to text sections.
> 
> The next patch will handle writes, which would only happen when the all
> the vmas with VM_DENYWRITE are unmapped.
> 
> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
> feature.
> 
> Acked-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  mm/Kconfig      | 11 ++++++
>  mm/filemap.c    |  4 +--
>  mm/khugepaged.c | 94 +++++++++++++++++++++++++++++++++++++++++--------
>  mm/rmap.c       | 12 ++++---
>  4 files changed, 100 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index f0c76ba47695..0a8fd589406d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -762,6 +762,17 @@ config GUP_BENCHMARK
>  
>  	  See tools/testing/selftests/vm/gup_benchmark.c
>  
> +config READ_ONLY_THP_FOR_FS
> +	bool "Read-only THP for filesystems (EXPERIMENTAL)"
> +	depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM

Hi,

Maybe a stupid question since I am new, but why does it depend on SHMEM?

Thanks,
-Kai